Introduce `around-cost` termination method.

Do some maths in the [lambda-function.py](https://github.com/jgreenemi/Parris/blob/master/src/lambda-function.py#L135) file based on the instance type in use to determine what the time-limit should be set to, so that the user can set a max dollar value that they are willing to pay for the training session.

Start in src/lambda-function.py around line 135 and trace how the instance type and training time-limit are currently handled. Define the inputs needed for an around-cost termination method, then verify that a user-supplied maximum dollar value is converted into a time limit appropriate to the selected instance type and ends the training session at that limit.

Written by the indexing model from the issue text.